But what happens if I buy a modern "digital only" 50mm lens? Does it equate to 50mm?

Click to expand...

The focal length of a lens is to do with it's physical construction, and has nothing to do with what it's mounted on. As such, a 50mm lens mounted on digital (Depending on the body - There's 1x 1.5x, 1.6x and 2x depending on brand / model) behaves like a 75mm, whether it's a modern "digital only" lens, or an old film lens.

Ok, so a 50mm "digital only" lens is completely physically identical to a classic 50mm lens then? But why are digital only lenses always 1.5 times smaller than their full-frame counterparts?

Man this stuff is weird.

Click to expand...

What is a "digital only" lens? What do you mean by "smaller"?

if you talk about focal length, a 50mm lens is always a 50mm no matter what medium it is built for. The size of the recording medium (film, large sensor, small sensor, whatever) will only influence the field of view which the 50mm lens will give you. A smaller medium will give a more narrow field of view than a larger medium.

correspondingly, the image circle (the area the image is projected onto) of lenses might vary. for a small sensor, that circle does not need to be as large as for a lager sensor or 35mm film. hence overall built size can be smaller and lighter.

This is why a 50mm medium format lens will be even larger, but again, it is a 50mm lens as well.

Let's say I have an old lens from a film camera; a 50mm. when I put it on my new camera, it works like a 75mm because of the 1.5 size difference of the sensor versus film. Ok. I get that.
But what happens if I buy a modern "digital only" 50mm lens? Does it equate to 50mm?

The focal length says at what distance behind the lens it would focus the pic' on a film or sensor.
The crop factor only says that the sensor covers a smaller area ot of the circular pic' that the lens projects inside the camera.

Lenses for digital cameras are the same, but made to project a smaller pic' circle.
This allows making the whole lens smaller.

With a 50mm film lens, on a 1.5 crop factor sensor, the full image that we get, is like what a 75mm film lens would have given on a film camera.

Just for an example, this is what a digital lens does on a film camera

The image projected on a digtal only lens is smaller, but the focal length is still the same. Using this lens on a digital camera you dont see the blacked out corners because the sensor is that much smaller than 35mm film.

Just wondering. What is the advantage for the most camera makers use the smaller sensor? Is it just because of the lower cost?

Click to expand...

Not just a lower cost, a much much much lower cost. One day I should do the exact maths on this but I'm sure it works out something like this. It costs a manufacturer to etch a wafer of sensors. APS sensors will allow at least twice if not more amount of sensors on the same cost wafer. Given that a wafer has a distribution of errors across it as the size of the sensor reduces the chances of a fault reduce and thus the yield per wafer increases as well.

So given that the sensor is only 1.5x smaller, it would cost them probably less than 1/4 of the full frame sensor to manufacturer.

I put my DA18-55 on my old K1000 while I was shooting that roll of B&W film the other day. It caused some vignetting, but it was equivalent to 12mm on the wide end and I can't wait to see how they turn out. I did some really fun architectural shots.